Method for producing adhesive polyacrylates using mercapto functional photoinitiators

ABSTRACT

A process for preparing pressure-sensitively adhesive polyacrylates by free-radical addition polymerization, characterized in that mercapto-functionalized photoinitiators of the general formula (I) and/or (II).  
     H—S—R   (I)  
     R—S—S—R′  (II)  
     are added to the monomer mixture or to the reaction mixture, R and R′ being chosen independently of one another from the following groups:  
     a) benzophenone, acetophenone, benzil, benzoin, hydroxyalkylphenone, phenylcyclohexyl ketone, anthraquinone, thioxanthone, triazine, or fluorenone radicals, it being possible for each of these radicals to be substituted by one or more halogen atoms and/or one or more alkoxy groups and/or one or more amino groups or hydroxyl groups,  
     b) radicals which comprise one or more of the substituted or unsubstituted radicals specified under a).

[0001] The invention relates to a process for preparingpressure-sensitively adhesive polyacrylates by free-radical additionpolymerization.

[0002] Pressure sensitive adhesives (PSAs) are increasingly beingemployed to produce PSA tapes, self-adhesive labels, adhering protectivefilms or other self-adhesive products. The PSAs employed for thesepurposes must have certain properties, such as, for example, goodsurface tack, high cohesion, good tack at low and high temperatures, andgood thermal load-bearing capacity.

[0003] DE 24 11 169 A1 describes UV-crosslinkable PSAs for whichcopolymers of (meth)acrylic esters, and, monoolefinically unsaturatedethers and (meth)acrylic ester derivatives of substituted benzophenoneare used as copolymerized photoinitiators. The UV reactivity of thepolymerizable benzophenones, however, is low, and after UV crosslinkingthe shear strength of the PSAs prepared from the polymers is too low.

[0004] U.S. Pat. No. 4,144,157 describes UV-crosslinkable PSAs whichhave been synthesized, for example, from acrylic esters and from(meth)acrylic acid 2-alkoxy-2-phenyl-2-benzoylethyl esters.Disadvantageous again is the low reactivity of these photoinitiators andthe associated low cohesion of the PSAs prepared.

[0005] U.S. Pat. No. 4,737,559 discloses UV-crosslinkable acrylic PSAscontaining copolymerizable (meth)acryloylbenzophenone derivatives in thepolymer chain. These PSAs were specifically conceived for use in themedical sector, such as for plasters, for example.

[0006] The PSAs prepared in accordance with said patent applicationrequire a relatively long irradiation time, as a result of which bondstrength and tack are adversely affected.

[0007] DE 38 44 445 A1 describes UV-crosslinkable PSAs based on(meth)acrylic ester polymers, which comprise a UV-reactive,copolymerized monomer in the form of an N-substituted(meth)acrylamide-benzophenone derivative; a(meth)acryloyloxy-benzophenone derivative or a styrene-benzophenonederivative.

[0008] UV-crosslinkable PSAs based on isoamyl (meth)acrylate copolymersare described in DE 38 36 968 A1. These PSAs are based onα-β-monoolefinically unsaturated acids whose homopolymers possess aglass transition temperature of below −30° C., on monoolefinicallyunsaturated acids and/or their anhydride, on further olefinicallyunsaturated monomers containing functional groups, and on apolymerizable (meth)acryloyloxybenzophenone or acetophenone derivative.Although the room temperature cohesion of the PSAs thus prepared;following brief UV irradiation, is acceptable, the thermal load-bearingcapacity is inadequate.

[0009] The thermal load-bearing capacity was solved in DE 195 01 024 A1through the use of a copolymerizable photoinitiator based on a diesterof carbonic acid.

[0010] All of the methods set out and described above, however, have akey disadvantage. The copolymerized photoinitiators fulfill only thefunction of UV crosslinking after coating. Accordingly it is necessary,for example, for preparing acrylic PSAs in acetone, to add regulators,so that the polymerization can be conducted with only a low solventfraction and gelling of the polymer is avoided. The reduction in thesolvent fraction is of great interest particularly forpolyacrylate-based hotmelt PSAs, since in this case, followingpolymerization, the solvent has to be removed, which is laborious, andit would therefore be desirable to minimize the amounts used, oneconomic and environmental grounds.

[0011] U.S. Pat. No. 5,942,555 used regulators likewise containing aphotoinitiator to prepare telechelic polymers, and subsequentlyactivated them with UV light.

[0012] Sulfur compounds in PSAs are known from the field of the rubberadhesives. There, they serve as stabilizers; see in this respect DE 19826 103 A1, which describes the use of solid monothiols, soluble ordispersible in rubber, as stabilizers in melted PSAs based on naturalrubber or synthetic rubbers, suitable tackifier resins, and promotersfor the purpose of increasing the radiation crosslinking yield.Polyfunctional (meth)acrylates can be added here as crosslinkingpromoters.

[0013] It is an object of the present invention to provide a process forpreparing UV-crosslinkable acrylic pressure sensitive adhesives,especially UV-crosslinkable acrylic hotmelt pressure sensitiveadhesives, with the acrylic PSAs thus prepared no longer having thedisadvantages mentioned of the prior art.

[0014] This object is achieved by means of a process as set out in themain claim. The subclaims relate to advantageous developments andembodiments of this process and also to the use of the adhesives thusprepared for producing PSA articles.

[0015] Claim 1 relates accordingly to a process for preparingpressure-sensitively adhesive polyacrylates by free-radical additionpolymerization, in which mercapto-functionalized photoinitiators of thegeneral formula (I) and/or (II)

H—S—R   (I)

R—S—S—R   (II)

[0016] are added to the monomer mixture or to the reaction mixture, Rand R′ being chosen independently of one another from the followinggroups:

[0017] a) benzophenone, acetophenone, benzil, benzoin,hydroxyalkylphenone, phenylcyclohexyl ketone, anthraquinone,thioxanthone,.triazine, or fluorenone radicals, it being possible foreach of these radicals to be substituted by one or more halogen atomsand/or one or more alkoxy groups and/or one or more amino groups orhydroxyl groups,

[0018] b) radicals which comprise one or more of the substituted orunsubstituted radicals specified under a).

[0019] In a first advantageous development of this process the fractionof the compounds (I) and/or (II) is from 0.01 to 5% by weight, based onthe monomers employed.

[0020] In a further embodiment of the process, which is very favorablein the inventive sense, the polymerization takes place using a monomermixture which comprises at least the following component:

[0021] a) acrylic and methacrylic acid monomers of the followingstructure

[0022] where R₁═H or CH₃

[0023] and R₂=an alkyl chain having 2-20 carbon atoms

[0024] with a fraction of from 65 to 100% by weight,

[0025] and which optionally comprises the following component:

[0026] b) vinyl compounds having functional groups

[0027] with a fraction of from 0 to 35% by weight,

[0028] it being possible optionally for further components to be presentin the monomer mixture.

[0029] Examples of monomers from group a) are butyl, pentyl, hexyl,heptyl, octyl, isooctyl, 2-methylheptyl, 2-ethylhexyl, nonyl, decyl,dodecyl, lauryl or stearyl (meth)acrylate or (meth)acrylic acid.Examples that may be mentioned of group b) include maleic anhydride,styrene, styrene compounds, vinyl acetate, (meth)acrylamides,N-substituted (meth)acrylamides, β-pacryloyloxypropionic acid,vinylacetic acid, fumaric acid, crotonic acid, aconitic acid,dimethylacrylic acid, trichloroacrylic acid, itaconic acid, vinylacetate, hydroxyalkyl (meth)acrylate, amino-containing (meth)acrylates,hydroxyl-containing (meth)acrylates, with particular preference2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate and/or4-hydroxybutyl (meth)acrylate, and double-bond-functionalizedphotoinitiators.

[0030] The composition of the corresponding monomers is preferablychosen such that the resultant adhesives possess pressure-sensitivelyadhesive properties in accordance with D. Satas [Handbook of PressureSensitive Adhesive Technology, 1989, VAN NOSTRAND REINHOLD, New York].

[0031] The free radical polymerization can be carried out in thepresence of one or more organic solvents and/or in the presence of wateror without solvent. It is preferred to use as little solvent aspossible. Depending on conversion and temperature, the polymerizationtime is between 6 and 48 h.

[0032] In the case of solution polymerization, solvents used includepreferably esters of saturated carboxylic acids (such as ethyl acetate),aliphatic hydrocarbons (such as n-hexane or n-heptane), ketones (such asacetone or methyl ethyl ketone), special boiling point spirit ormixtures of these solvents. For polymerization in aqueous media or inmixtures of organic and aqueous solvents, it is preferred to addstabilizers and emulsifiers which are familiar to the skilled worker forthis purpose to the polymerization. Polymerization initiators usedinclude customary radical-forming compounds such as peroxides, azocompounds, and peroxosulfates, for example. Initiator mixtures can alsobe used.

[0033] In the polymerization, in addition to the thio compounds modifiedwith a UV photoinitiator, it is also possible to use further regulatorsfor lowering the molecular weight and reducing the polydispersity. Asso-called polymerization regulators it is possible, for example to usealcohols and ethers.

[0034] The polymerization may be carried out in polymerization reactors,which are generally provided with a stirrer, two or more feed vessels,reflux condenser, heating and cooling, and are equipped for operationunder N₂ atmosphere and superatmospheric pressure.

[0035] Following polymerization in solvent, the polymerization mediumcan be rermoved under reduced pressure, this operation being carried outat elevated temperatures, for example, and advantageously in the rangefrom 80 to 150° C. The polymers can then be used in solvent-free state,particularly as hotmelt PSAs. Depending on further processing and fieldof application it may also be of advantage to prepare the polymers ofthe invention without solvent.

[0036] To prepare the acrylic PSAs, the polymers of the invention can bemodified in customary fashion. Added favorably, for example, aretackifying resins, such as terpene, terpenephenolic, C5, C9, C5/C9hydrocarbon, pinene or indene resins or rosins, alone or in combinationwith one another. Depending on application it is further of advantage toadmix plasticizers, various fillers (e.g., chalk, carbon black, glassmicrobeads, etc.) and aging inhibitors as additives. Optionally,furthermore, crosslinkers, and UV crosslinking promoters that are knownto the skilled worker are admixed. Polyfunctional acrylate crosslinkersare particularly advantageous for the preparation of star polymers.

[0037] The polymers are advantageously applied conventionally bybrushing, spraying, rolling, knifecoating, pouring or extruding, whereappropriate at elevated temperature—usually in the temperature rangefrom 20 to 150° C.—to substrates suitable as carrier materials for thispurpose: for example, to paper, paperboard, wood, metals, and polymerfilms, made for example from plasticized PVC; polyethylene, polyamides,polyethylene glycol terephthalate or polypropylene. Where solvents areused, they can easily be evaporated from the coatings, where appropriateat room temperature or slightly elevated temperatures, generally attemperatures of 20-150° C., preferably of 50-100° C., using,customarily, radiant heaters or heated-air circulation apparatus.

[0038] In a way which is also very advantageous for the inventiveprocess, the pressure-sensitively adhesive polyacrylates are crosslinkedby ultraviolet radiation in a wavelength range from 200 to 400 nm.Crosslinking of the hotmelt PSAs of the invention takes placeadvantageously by brief UV irradiation with commercially customaryhigh-pressure or medium-pressure mercury lamps having an output of, forexample, from 80 to 160 W/cm. It may be appropriate to adapt the outputof the lamp to the belt speed or, if the belt is running slowly, toshade off the belt partly in order to reduce the thermal load thereon.The irradiation time depends on the construction and output of therespective lamps.

[0039] The invention relates, moreover, to the use of thepressure-sensitively adhesive polyacrylates prepared by the inventiveprocess to produce PSA articles, particularly for producing an adhesivetape comprising an acrylic PSA applied to one or both sides of abacking.

[0040] The UV-curing polyacrylates prepared by the inventive process areparticularly suitable as melts or as solutions for preparing PSAmaterials having improved tack, improved bond strength, and highcohesion, such as PSA tapes, PSA sheets or PSA labels.

[0041] The PSA properties of the polyacrylates prepared by the inventiveprocess, following UV irradiation, are determined by the test methodsdescribed below.

[0042] For the test, films of polyethylene glycol terephthalate werecoated with an adhesive application of 50 g/m².

[0043] Where dissolved polymers are used for adhesive performancetesting, the solvents are evaporated in a drying oven at 120° C. for 10minutes. The dry PSA films were irradiated using an Eltosch unit withmedium-pressure mercury lamps. The distance of the UV lamps from theirradiated PSA films was 5 cm, the output of the UV lamps is 120 W/cm.

[0044] The invention is illustrated below by examples. Amounts,proportions, and percentages are based on the total amount of themonomers.

[0045] Preparation of the UV-Activatable Thior Gulators

[0046] Overview of the thio-functionalized photoinitiators prepared (seetable 1) TABLE 1 Thio-functionalized photoinitiators

Photoinitiator (I)

Photoinitiator (II)

Photoinitiator (III)

Photoinitiator (IV)

EXAMPLE 1

[0047] Preparation of 4-benzoylbenzoyl chloride

[0048] 1.0 kg (4.2 mol) of 4-benzoylbenzoic acid were filled into a 5 Lreactor, equipped with a reflux condenser and a stirrer, andadditionally 645 ml (8.84 mol) of thionyl chloride and 725 ml of toluenewere added. Then 3.5 ml of DMF were added and the mixture was boiledunder reflux for 4 h. After cooling, the solvent was removed underreduced pressure and excess thionyl chloride was removed by evaporatingthree times with 500 ml of toluene each time. The product wasrecrystallized from a 1:4 toluene/hexane mixture and recovered with 934g (86% yield) after-drying in a vacuum oven.

[0049]¹H-NMR at 300 MHz (CDCl₃) gave: 7.18-8.26 ppm (m, 9 H). Thesignals were consistent with the desired product. All chemical shiftsare stated with respect to tetramethylsilane as internal standard.

EXAMPLE 2

[0050] Preparation of 4-bromomethylbenzophenone

[0051] 750 g (3.82 mol) of 4-methylbenzophenone were filled into a 5 Lreactor, equipped with a reflux condenser and a stirrer, andadditionally 2850 ml of benzene were added. The mixture was heated toreflux and 610 g (3.82 mol) of bromine in solution in 330 ml of benzenewere added dropwise. The rate of addition was approximately 1.5 ml/min.The reactor was irradiated with a halogen lamp at 100 W to initiate thereaction, thereafter with different cycles of identical output.

[0052] In each of the cycles, the irradiation was carried out inalternation for a period of 5 s, followed by a period of 40 s withoutirradiation. After 1 h, this cycle was altered to a sequence of 10 sirradiation and 40 s without irradiation. After the end of the reaction(decoloration of the dark brown solution by consumptive reaction withbromine) the crude product was analyzed by means of GC. A mixture ofmonobromo and dibromomethyl-benzophenone and unreacted4-methylbenzophenone was found. The reaction mixture was washed with 10g of sodium thiosulfite in 100 g of water and 3 times with 200 g ofwater each time. The product was subsequently dried over sodium sulfateand recrystallized twice from 1:3 toluene/hexane. After drying underreduced pressure, 590 g of 4-bromomethylbenzophenone (56% yield) wereisolated.

[0053]¹H-NMR at 300 MHz (CDCl₃) gave [ppm]: 7.18-7.77 (m, 9 H), 4.49 (s,2H). The signals were consistent with the desired product.

EXAMPLE 3

[0054] Preparation of 4-mercaptomethylbenzophenone (I)

[0055] 4.14 g (54.4 mmol) of thiourea were dissolved in 31.5 ml ofethanol (95%), 15.0 g (54.4 mmol) of 4-bromomethylbenzophenone wereadded with gentle heating and stirring, and the mixture was stirredovernight at room temperature. The solid product was isolated byfiltration and washed several times with ethanol. After drying in avacuum oven, 15.6 g were isolated (82% yield). No further purificationwas carried out.

[0056] 12.5 g (35.5 mmol) of this hydrobromide salt were dissolved in250 ml of water with heating and then 5.7 g of sodium hydroxide (0.143mol) in solution in 10 ml of water were added. After 45 minutes ofrefluxing the solution was cooled to room temperature, a pH of less than2 was set using concentrated sulfuric acid, and the product wasextracted 5 times with 60 ml of chloroform. The combined extracts werewashed with 100 ml of water and dried over sodium sulfate. After thesolvent had been removed, 7.9 g (97%) were isolated.

[0057] The melting point was 54° C. ¹H-NMR at 300 MHz (CDCl₃) gave[ppm]: 7.14-7.78 (m, 9 H), 3.70 (d, 2H) and 1.77 (t, 1 H). The signalswere consistent with the desired product.

EXAMPLE 4

[0058] Preparation of N-(2-mercaptoethyl)-4-benzoylbenzamide (II)

[0059] 24.39 g (0.215 mol) of 2-aminoethanethiol hydrochloride insolution in 200 ml of chloroform were placed under argon in a 1 Lthree-necked flask. A solution of 50 g (0.204 mol) of 4-benzoylbenzoylchloride and 250 ml of chloroform was subsequently added dropwise over aperiod of 45 minutes. The mixture was stirred at room temperatureovernight. It was then washed with water and a 0.1 N HCl solution anddried over sodium sulfate. After the drying operation, twofoldrecrystallization from toluene gave 50 g of a white powder (86% yield).

[0060] The melting point was 112° C. ¹H-NMR at 300 MHz (CDCl₃) gave[ppm]: 7.18-7.82 (m, 9 H), 6.70-7.02 (m, 1H), 3.52 (q, 2H), 2.54-2.97(m, 2H) and 1.37 (t, 1H). The signals were consistent with the desiredproduct.

EXAMPLE 5

[0061] Preparation ofN-(2-mercaptoethyl)-3,5-bis(4-benzoylbenzoyloxy)benzamide (III)

[0062] 46.2 g (0.30 mol) of 3,5-dihydroxybenzoic acid were introducedinto a 250 ml flask with Soxlett extractor and reflux condenser. 48.6 mlof methanol and 0.8 ml of sulfuric acid were added and 50 g of molecularsieve (3Å) were placed in the Soxlett extractor. The extractor wasfilled with methanol and the entire mixture was refluxed overnight. Themethylated crude product was subsequently isolated after removal of thesolvent.

[0063] The entire product was placed in a 2 L reactor with refluxcondenser and stirrer, and then 173.25 g (0.63 mol) of4-bromomethylbenzophenone, 207 g (1.50 mol) of potassium carbonate and1200 ml of acetone were added. During overnight reflux, the reaction wasmonitored by thin-layer chromatography. Following complete conversionthe solid was isolated by filtration and acetone was removed underreduced pressure. The solid was dissolved in 1 L of water and thenextracted with three times 1 L of chloroform. The extracts were combinedwith the acetone-soluble fraction and dried over sodium sulfate, and 177g of the crude product were isolated. The crude product wasrecrystallized twice from acetonitrile, to give ultimately 145 g (87%).

[0064] The melting point was 130° C. ¹H-NMR at 300 MHz (CDCl₃) gave[ppm]: 7.22-7.78 (m, 18 H), 7.15 (d, 2H), 6.69 (t, 1H), 5.02 (s, 4H) and3.84 (s, 3H). The signals were consistent with the desired product:methyl 3,5-bis(4-benzoylbenzoyloxy)benzoate.

[0065] 60.1 g (0.108 mol) of methyl 3,5-bis(4-benzoylbenzoyloxy)benzoatewere introduced into a 2 L reactor and 120 ml of water, 480 ml ofmethanol and 6.48 g (0.162 mol) of sodium hydroxide were added. Thereaction mixture was refluxed for 3 h. After hydrolysis of the ester,the mixture was cooled and methanol was removed under reduced pressure.The sodium salt which remained was dissolved in 2 400 ml of hot waterand the free acid was subsequently precipitated using hydrochloric acid.Filtration, washing with water, and vacuum oven drying gave 54 g of awhite powder (92%).

[0066] The melting point was 188° C. The melting point was 130° C.¹H-NMR at 300 MHz (CDCl₃) gave [ppm]: 7.28-7.78 (m, 18 H), 7.15 (d, 2H),5.86 (t, 1H) and 5.16 (s, 4H). The signals were consistent with thedesired product: 3,5-bis(4-benzoylbenzoyloxy)benzoic acid.

[0067] 20 g (36.86 mmol) of 3,5-7bis(4-benzoylbenzoyloxy)benzoic acidwere introduced together with 36 ml of toluene, 5.4 ml (74.0 mmol) ofthionyl chloride and 28 μl of N,N-dimethylformamide into a 250 ml flask.The mixture was refluxed for 4 h. After the acid chloride had formed themixture was cooled to room temperature, the solvent and excess thionylchloride were removed under reduced pressure, and purification wascontinued by evaporating a further 4 times with 20 ml of chloroform eachtime. Recrystallization from toluene gave 18.5 g of the product (89%yield).

[0068] The melting point was 125° C. ¹H-NMR at 300 MHz (CDCl₃) gave[ppm]: 7.29-7.78 (m, 18 H), 7.20 (d, 2H), 6.79 (t, 1H), 5.08 (s, 4H).The signals were consistent with the desired product:3,5-bis(4-benzoylbenzoyloxy)benzoyl chloride.

[0069] 4.19 g (36.7 mmol) of 2-aminoethanethiol hydrochloride wereintroduced into a 250 ml flask with reflux condenser and stirrer andthen 15 ml of chloroform and 10.64 ml (76.5 mmol) of triethylamine wereadded. The reaction mixture was cooled to 0° C. using an ice bath andthen 18.4 g (32.8 mmol) of 3,5-bis(4-benzoylbenzoyloxy)benzoyl chloridein solution in 50 ml of chloroform were added dropwise over a period of50 minutes. After a further 30 minutes of ice cooling, the mixture waswarmed to room temperature for 2 h. The product was diluted with 150 mlof chloroform, washed with 5 times 250 ml of 0.1 N hydrochloric acid,dried over sodium sulfate and finally recrystallized twice from 15:1toluene/hexane. 12.9 g (65% yield) of the product were isolated.

[0070] The melting point was 114° C. ¹H-NMR at 300 MHz (DMSO-d6) gave[ppm]: 7.20-7.80 (m, 18 H), 7.00 (d, 2H), 6.66 (t, 1H), 6.52 (broad t,1H), 5.08 (s, 4H), 3.50 (q, 2H), 2.74 (q, 2H) and 1.40 (t, 1 H). Thesignals were consistent with the desired product:N-(2-mercaptoethyl)-3,5-bis(4-benzoylbenzoyloxy)benzamide. (III).

EXAMPLE 6

[0071] Preparation ofN-(2-mercaptoethyl)-2,6-bis(4-benzoylbenzamido)hexanamide (IV)

[0072] 3.65 g (20 mmol) of lysine monohydrochloride were dissolved in 8ml of 2 N aqueous sodium hydroxide solution and the solution was cooledusing an ice bath. A solution of 10.77 g (44 mmol) of 4-benzoylbenzoylchloride in 17 ml of chloroform was added simultaneously with 4.48. ofsodium hydroxide in 19 ml of water. The reaction mixture was stirred for2 h with ice cooling and then for 3 h at room temperature. Hydrochloricacid was used to set a pH of less than 1, and then 60 ml of chloroformwere added. The different phases were separated with a centrifuge andthe aqueous phase was extracted with three times 50 ml of chloroform.The combined organic phases were dried over sodium sulfate. The motherliquor was diluted with water and the precipitated product was filteredoff, dissolved again in chloroform and then washed with 10% strengthaqueous sodium hydrogen carbonate solution, 1 N hydrochloric acid andwater. The product was used without further purification.

[0073] 4.35 g (7.73 mmol) of the lysine derivative and 0.901 g (7.83mmol) of N-hydroxy-succinimide were dissolved in 40 ml of 1,4-dioxane,and then 1.951 g (9.45 mmol) of 1,3-dycyclohexylcarbodiimide (DCC) in 10ml of 1,4-dioxane were added. The ester was filtered off and dried (4.1g, 81% yield). In a separate flask, 0.75 g (6.6 mmol) of2-aminoethanethiol hydrochloride were dissolved in 15 ml of chloroform.4.1 g (6.22 mmol) of the ester, following dissolution in 25 ml ofchloroform, are added slowly dropwise to this solution at roomtemperature over a period of 30 minutes. After 4 h the reaction mixturewas washed with water and 0.05 N hydrochloric acid, dried over sodiumsulfate and then purified by column chromatography using a solventmixture of 95% chloroform and 5% methanol. 2.2.g (yield 56%) of theproduct were isolated.

[0074]¹H-NMR at 300 MHz (CDCl₃) gave [ppm]: 6.90-7.95 (m, 21 H),4.42-4.87 (m, 1 H), 3.00-3.78 (m, 4H), 1.42 (t, 1 H) and 1.00-2.95 (m,8H). The signals were consistent with the desired product:N-(2-mercaptoethyl)-2,6-bis(4-benzoylbenzamido)hexanamide (IV).

[0075] Polymerizations

[0076] 180° Bond Strength Test (Test Method A)

[0077] A strip 20 mm wide of an acrylic PSA laminated onto a polyesterfilm was applied to steel plates washed twice with acetone and once withisopropanol. The PSA strip was pressed onto the substrate twice using a2 kg weight. The adhesive tape was then immediately peeled from thesubstrate at a speed of 300 mm/min and at an angle of 180°. All.measurements were conducted at room temperature under standardizedclimatic conditions.

[0078] The measurement results are reported in N/cm and are averagedfrom three measurements.

[0079] Shear Strength (Test Method B)

[0080] A strip of the adhesive tape 13 mm wide was applied to a smoothsteel surface cleaned three times with acetone and once withisopropanol. The area of application was 20 mm×13 mm (length×width). Theadhesive tape was then pressed onto the steel substrate four times usinga 2 kg weight. A 1 kg weight was fastened to the adhesive tape at roomtemperature and at 70° C.

[0081] The shear stability times measured are reported in minutes andcorrespond to the average of three measurements.

EXAMPLE 7

[0082] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butylacrylate, 20 g of acrylic acid, 200 g of acetone and 4 g of4-mercaptomethylbenzophenone. After 30 minutes of nitrogen gasinertization, the reaction mixture was heated to an internal temperatureof 58° C. with stirring and then 0.2 g of AIBN (azoisobutyronitrile) wasadded 0.2 g of AIBN was added after a reaction time of 1.5 h, and 100 gof acetone after 3 h. After 8 and 10 hours of reaction, in each case 0.2g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate) wasadded, and after 10 h a further 100 g of acetone were added fordilution. The polymerization was terminated after 24 h by cooling. Theproduct was a polymer having a molecular weight of 740 000 g/mol (M_(w)from gel permeation chromatography). The PSA thus prepared was diluteddown to 35% (solids) with acetone and then applied at 50 g/m² (solidsafter drying) to a primed PET (polyethylene terephthalate) film 23 μmthick. The PSA tape was dried in a drying oven at 120° C. for 10minutes. The PSA specimen was subsequently irradiated in a UV unit (fromEltosch) with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.

[0083] The UV-crosslinked PSA tapes were tested for bond strength onsteel (test method A) and for cohesion by a shear test at roomtemperature and at 70° C. (test method B).

[0084] The results are compiled in table 2. TABLE 2 SST, RT, 10 N SST,70° C., 10 N BS steel [N/cm] [min] [min] Example 7 6.1 7 840 1 355

EXAMPLE 8 (COMPARATIVE EXAMPLE)

[0085] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butylacrylate, 20 g of acrylic acid, 200 g of acetone and 4 9 of Ebecryl© P36 (acrylated benzophenone derivative from UCB). After 30 minutes ofnitrogen gas inertization, the reaction mixture was heated to aninternal temperature of 58° C. with stirring and then 0.2 g of AIBN(azoisobutyronitrile) was added. 0.2 g of AIBN was added after areaction time of 1.5 h, 100 g of acetone after 3 h. After a reactiontime of 8 h, 0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl)peroxydicarbonate) was added.

[0086] After 9 h, the polymerization had to be terminated, since thebatch had completely gelled.

EXAMPLE 9 (COMPARATIVE EXAMPLE)

[0087] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 320 g of 2-ethylhexyl acrylate, 56 g of butylacrylate, 20 g of acrylic acid, 200 g of acetone/isopropanol (97:3) and4 g of Ebecryl© P 36 (acrylated benzophenone derivative from UCB). After30 minutes of nitrogen gas inertization, the reaction mixture was heatedto an internal temperature of 58° C with stirring and then 0.2 g of AIBN(azoisobutyronitrile) was added. 0.2 9 of AIBN was added after areaction time of 1.5 h, and 100 g of acetone/isopropanol (97:3) after 3h. After 8 and 10 hours of reaction, in each case 0.2 g of Perkadox© 16(bis(4-tert-butylcyclohexanyl) peroxydicarbonate) was added, and after10 h a further 100 g of acetone/isopropanol (97:3) were added fordilution. The polymerization was terminated after 24 h by cooling. Theproduct was a polymer having a molecular weight of 760 000 g/mol (M_(w)from gel permeation chromatography). The PSA thus prepared was diluteddown to 35% (solids) with acetone and then applied at 50 g/m² (solidsafter drying) to a primed PET (polyethylene terephthalate) film 23 μmthick. The PSA tape was dried in a drying oven at 120° C. for 10minutes. The PSA specimen was subsequently irradiated in a UV unit (fromEltosch) with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.

[0088] The UV-crosslinked PSA tapes were tested for bond strength onsteel (test method A) and for cohesion by a shear test at roomtemperature and at 70° C. (test method B).

[0089] The results are compiled in table 3. TABLE 3 SST, RT, 10 N SST,70° C., 10 N BS steel [N/cm] [min] [min] Example 10 5.8 6 350 560

EXAMPLE 10

[0090] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylicacid, 200 g of acetone and 6 g ofN-(2-mercaptoethyl)-4-benzoylbenzamide. After 30 minutes of nitrogen gasinertization, the reaction mixture was heated to an internal temperatureof 58° C. with stirring, and subsequently the procedure of example 7 wasfollowed. The product is a polymer having a molecular weight of 710 000g/mol (M_(w) from gel permeation chromatography). The PSA thus preparedwas diluted down to 35% (solids) with acetone and then applied at 50g/m² (solids after drying) to a primed PET (polyethylene terephthalate)film 23 μm thick. The PSA tape was dried in a drying oven at 120° C. for10 minutes. The PSA specimen was subsequently irradiated in a UV unit(from Eltosch) with a UV lamp (120 W/cm, 254,nm) in 2 passes at 20m/min.

[0091] The UV-crosslinked PSA tapes were tested for bond strength onsteel (test method A) and for cohesion by a shear test at roomtemperature and at 70° C. (test method B).

[0092] The results are compiled in table 4. TABLE 4 SST, RT, 10 N SST,70° C., 10 N BS steel [N/cm] [min] [min] Example 10 5.2 +10 000 7 525

EXAMPLE 11 (COMPARATIVE EXAMPLE)

[0093] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylicacid, 200 g of acetone and 6 g of benzoin acrylate (prepared by themethod of Guse et al. in accordance with DE 27 43 979 A1). After 30minutes of nitrogen gas inertization, the reaction mixture was heated toan internal temperature of 58° C. with stirring and then 0.2 g of AIBN(azoisobutyronitrile) was added 0.2 g of AIBN was added after a reactiontime of 1.5 h, 100 g of acetone after 3 h. After a reaction time of 8 h,0.2 g of Perkadox© 16 (bis(4-tert-butylcyclohexanyl) peroxydicarbonate)was added.

[0094] After 9 h, the polymerization had to be terminated, since thebatch had completely gelled.

EXAMPLE 12 (COMPARATIVE EXAMPLE)

[0095] A conventional 2 L glass reactor equipped with a mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath was charged with 360 g of 2-ethylhexyl acrylate, 34 g of acrylicacid, 200 g of acetone/isopropanol (97:3) and 6 g of benzoin acrylate(prepared by the method of Guse et al. in accordance with DE 27 43 979A1). After 30 minutes of nitrogen gas inertization, the reaction mixturewas heated to an internal temperature of 58° C. with stirring, andsubsequently the procedure of example 7 was followed. The product was apolymer having a molecular weight of 750,000 g/mol (M_(w) from gelpermeation chromatography). The PSA thus prepared was diluted down to35% (solids) with acetone. and then applied at 50 g/m² (solids afterdrying) to a primed PET (polyethylene terephthalate) film 23 μm thick.The PSA tape was dried in a drying oven at 120° C. for 10 minutes. ThePSA specimen was subsequently irradiated in a UV unit (from Eltosch)with a UV lamp (120 W/cm, 254 nm) in 2 passes at 20 m/min.

[0096] The UV-crosslinked PSA tapes were tested for bond strength onsteel (test method A) and for cohesion by a shear test at roomtemperature and at 70° C. (test method B).

[0097] The results are compiled in table 5. TABLE 5 SST, RT, 10N SST,70° C., 10 N BS steel [N/cm] [min] [min] Example 9 5.3 +10 000 2 455

[0098] As apparent from the described examples 7 to 12, themercapto-functionalized photo-initiator is very suitable for preparingacrylic PSAs by the inventive process. There is no need to supply anyregulator to the polymerization (examples 7+10). The polymerizationsconducted analogously (examples 8 and 11) gelled. Comparison withexamples 9 and 12 shows that acrylated photoinitiators can becopolymerized by the addition of a regulator. Comparison of tables 2 and3 and 4 and 5, respectively, indicates a significantly poorer thermalshear stability for this method, however.

EXAMPLES 13 TO 18

[0099] The polymerization of the following monomer mixtures (table 6:composition of the monomer mixtures used; amounts in % by weight) wasconducted in a conventional 2 L glass reactor equipped with mechanicalstirrer, reflux condenser, internal temperature sensor and a heatingbath. The procedure adopted was analogous to that in example 7,retaining the amounts of solvent and initiator. There was no changeeither in the times at which each of the additions were made. Thereaction time was 24 h.

[0100] The polymers prepared by the process of the invention werediluted down to 35% (solids) with acetone and then applied at 50 g/m²(solids after drying) to a primed PET (polyethylene terephthalate) film23 μm thick. The PSA tapes were dried in a drying oven at 120° C. for 10minutes. The PSA specimens were subsequently irradiated in a UV unit(from Eltosch) with a UV lamp (120 W/cm, 254 nm),in 2 passes at 20m/min.

[0101] The UV-crosslinked PSA tapes were tested for bond strength onsteel (test method A) and for cohesion by a shear test at roomtemperature and at 70° C. (test method B).

[0102] The results are compiled in table 7. TABLE 6 Mercapto-functionalized photoinitiator 2-EHA AA 2-HEA IO MA Conc. Example [%] [%][%] [%] [%] Comp. [%] 13 0 5 0 75 19 I 1 14 46 1 5.5 46 0 II 1.5 15 0 50 75 18 III 2 16 88 10 0 0 0 IV 2 17 95 3 0 0 0 I 2 18 60 10 0 20 8 I 2

[0103] TABLE 7 SST, RT, 10 N SST, 70° C., 10 N Example BS steel [N/cm][min] [min] 13 4.8 +10 000 6 890 14 5.9    2 325    280 15 5.0 +10 000   7 275 16 4.6 +10 000 +10 000 17 5.3    5 640    1 065 18 4.4 +10 000+10 000

1. A process for preparing pressure-sensitively adhesive polyacrylatesby free-radical addition polymerization, characterized in thatmercapto-functionalized photoinitiators of the general formula (I)and/or (II) H—S—R   (I) R—S—S—R′  (II) are added to the monomer mixtureor to the reaction mixture, R and R′ being chosen independently of oneanother from the following groups: a) benzophenone, acetophenone,benzil, benzoin, hydroxyalkylphenone, phenylcyclohexyl ketone,anthraquinone, thioxanthone, triazine, or fluorenone radicals, it beingpossible for each of these radicals to be substituted by one or morehalogen atoms and/or one or more alkoxy groups and/or one or more aminogroups or hydroxyl groups, b) radicals which comprise one or more of thesubstituted or unsubstituted radicals specified under a).
 2. The processof claim 1, characterized in that the fraction of the compounds (I)and/or (II) is from 0.01 to 5% by weight, based on the monomersemployed.
 3. The process of at least one of the preceding claims,characterized in that the polymerization takes place using a monomermixture which comprises at least the following component: a) acrylic andmethacrylic acid monomers of the following structure

where R₁═H or CH₃ and R₂=an alkyl chain having 2-20 carbon atoms with afraction of from 65 to 100% by weight, and which optionally comprisesthe following component: b) vinyl compounds having functional groupswith a fraction of from 0 to 35% by weight, it being possible optionallyfor further components to be present in the monomer mixture.
 4. Theprocess of at least one of the preceding claims, characterized in thatthe pressure-sensitively adhesive polyacrylates are crosslinked byultraviolet radiation in a wavelength range from 200 to 400 nm.
 5. Theuse of the pressure-sensitively adhesive polyacrylates of at least oneof the preceding claims to produce PSA articles, particularly forproducing an adhesive tape comprising an acrylic PSA applied to one orboth sides of a backing.